Probing Defect States in Organic Polymers and Bulk Heterojunctions

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C: Energy Conversion and Storage; Energy and Charge Transport

Probing Defect States in Organic Polymers and Bulk Heterojunctions Using Surface Photovoltage Spectroscopy Lakshmi N. S. Murthy, Diego Barrera, Liang Xu, Aakash Gadh, FongYi Cao, Cheng-Chun Tseng, Yen-Ju Cheng, and Julia W.P. Hsu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01667 • Publication Date (Web): 10 Apr 2019 Downloaded from http://pubs.acs.org on April 10, 2019

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The Journal of Physical Chemistry

Probing Defect States in Organic Polymers and Bulk Heterojunctions using Surface Photovoltage Spectroscopy Lakshmi N.S. Murthy†, Diego Barrera†, Liang Xu†, Aakash Gadh†, Fong-Yi Cao‡, Cheng-Chun Tseng‡, Yen-Ju Cheng‡, Julia W.P. Hsu†*

†Department

of Materials Science and Engineering, University of Texas at Dallas, Richardson, TX, 75080 USA

‡Department

of Applied Chemistry, National Chiao Tung University, 1001 University Road, Hsinchu, 30010 Taiwan

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ABSTRACT: We performed frequency modulated (AC) and steady state (DC) surface photovoltage spectroscopy (SPS) measurements on a bilayer structure consisting of an organic semiconductor (P3HT, P3HT:PC61BM, and PFBT2Se2Th:PC71BM) on top of a ZnO electron transport layer. The AC spectra overlap with the absorption spectra of the organic layer, evident that AC SPS corresponds to band-to-band transitions. The DC spectra are generally broader than AC spectra, with responses extended below the absorption edge. Thus, DC SPS also probes transitions between band states and trap states within the bandgap in addition to band-to-band transitions. When a hole transport layer (HTL) was deposited on top of the organic layer, the DC spectra of P3HT and P3HT:PC61BM are narrower than those without the HTL, suggesting that the sub-bandgap states exist at the surface of these organic semiconductors. In contrast, PFBT2Se2Th:PC71BM does not show signature of surface states or optically active trap states in the band gap. External quantum efficiency and capacitance measurements are employed to explain the nature of sub-band gap states that contribute to surface photovoltage signals and the differences between the two bulk heterojunction systems.


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1. INTRODUCTION In recent years, organic photovoltaics (OPV) has attracted a great deal of attention due to the low cost of fabrication, light weight, and mechanical flexibility.1,2 The hole transport layer (HTL) and electron transport layer (ETL) sandwiching the active layer in OPV devices establish the built-in field, prevent carrier recombination, and aid charge transport to the metal electrodes.3 The charge transfer from organic absorber layer to carrier transport layer is one of the critical step of carrier collection in OPV devices.4 Defect states in the bandgap of organic semiconductor can impede the charge collection process by limiting exciton diffusion,5 and introducing carrier traps or recombination center.6,7 Therefore, studying sub-bandgap defect states in the organic layer is critical for understanding device physics. Defect studies are typically performed using capacitance-frequency (C-f) analysis,8,9 deep-level transient spectroscopy,10 or thermally stimulated currents.11 A drawback common to these techniques is that they require two contacts, usually on the top and bottom of the organic layer. Specifically, deposition of top contact can perturb the organic semiconductor, e.g. creating defects or causing reactions at that interface.12,13,14 Here we study sub-bandgap states in organic 3 ACS Paragon Plus Environment


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semiconductors using surface photovoltage spectroscopy (SPS), which only requires the organic layer to be deposited on a conducting substrate.

SPS is a well-established, non-contact characterization technique for analyzing photoinduced charge separation and electrically active defects in semiconductors.15,16 Surface photovoltage (SPV) is defined as the contact potential difference under illumination minus that in the dark.15 The sign of SPV indicates the type of charge carriers transferred,15 and the magnitude of SPV has been shown to be directly proportional to open-circuit voltage (Voc) of the corresponding solar cells,17,18,19 hence reflecting to the quasi-Fermi level splitting. SPS measures SPV as a function of incident photon energy. SPS results can provide quantitative information on bulk electronic properties (e.g. bandgap and defect state energies) and to construct surface and interface band diagrams.15,16 Note that SPS differs from absorption spectrum in that not only does a photon have to be absorbed by the sample, charge separation has to occur to generate a SPV signal.15 Moreover, sub-bandgap illumination can probe the transition between band states and defect states within the bandgap. SPS has been applied to


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various inorganic semiconductors,20,21,22 organic-inorganic halide perovskites,23,24 small molecules25 and pure polymer,26,27 and organic bulk heterojunction (BHJ)18,28 films. There are also quite a few scanning kelvin probe microscopy (SKPM) studies of phase separation in organic BHJ blends,29,30 but SKFM studies typically focus on lateral variation. In this study, we focus on spectral variation of different organic semiconductors and the effect of surface defects.

2. EXPERIMENTAL DETAILS

2.1 Sample Preparation

Figure 1(a) shows the schematic diagram of SPV measurement on the sample structure used in this study: an organic semiconductor on top of a zinc oxide (ZnO) film on top of an ITO film on glass, which is the same as the bottom half of an inverted OPV device. The 30-nm ZnO layer was deposited onto precleaned ITO (Xinyan, 15 Ω/sq) substrates by a sol-gel method, using 0.5 M zinc acetate dihydrate and 0.5 M ethanolamine dissolved in 2-methoxyethanol; annealing was performed at 170 °C in air followed by water, isopropyl alcohol, and acetone rinsing.31 We study three different organic 5 ACS Paragon Plus Environment


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semiconductors: poly(3-hexylthiophene) (P3HT), P3HT: [6:6]-phenyl C61-butyric acid methyl ester (P3HT:PC61BM), and poly(5-bromo-4-(2-octyldodecyl)selenophen2-yl)-5,6difluorobenzothiadiazole-5,5’-bis(trimethylstannyl)- 2,2’-bithiophene: [6,6]-phenyl C71butyric acid methyl ester (PFBT2Se2Th:PC71BM). P3HT (Rieke) (20 mg/ml) and P3HT:PC61BM (Solenne BV) (20:20 mg/ml) were dissolved in dichlorobenzene (DCB) (Sigma Aldrich) and stirred overnight at 60 °C. The organic semiconductor films are formed by spin-coating the DCB solution at 600 rpm for 60 s, kept in a covered petri dish for 4 h (solvent annealing), and followed by 120 °C annealing in N2 for 10 min. Using previously published receipe,32 PFBT2Se2Th:PC71BM (6:12 mg/ml) with 5 vol % diphenyl ether as the additive was dissolved in DCB solution and stirred overnight at 100 °C. This solution and ITO/ZnO substrate were preheated at 100 °C, and the film is formed by spin-coating at 1000 rpm for 60 s. The coated substrate was placed in the vacuum chamber (~ 60 s) to dry. We also investigated the effects of the top HTL. For P3HT and P3HT:PC61BM, 30 nm of poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) (Heraeus Clevios AI 4083) was spin-coated on


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top of the organics followed by 120 °C anneal. Because PFBT2Se2Th:PC71BM degrades when heated, we thermally evaporated 7 nm MoO3 as HTL.

2.2 Measurements

SPV signal was measured with kelvin probe (SKP5050, KP Technology) in air over the organic semiconductor layer with light from a tungsten lamp illuminating from the ITO side, as shown in Figure 1(a). The wavelength of monochromatic light in SPS was selected using two linear variable filters to cover the range 350-1100 nm. Two types of SPS measurements were performed in this study (Figure 1(b)): AC SPS was performed with a stationary Kelvin probe tip and monochromatic light illumination modulated at a specified frequency, and DC SPS was performed with a vibrating Kelvin probe under continuous monochromatic light illumination. The DC SPS data were collected by varying the light wavelength from long to short, i.e increasing photon energy; this procedure avoids initially filling sub-bandgap states due to above-bandgap excitation that have not relaxed completely. In contrast, non-zero AC SPV signals arise only when the photogenerated charges are able to respond to the fast modulation of chopped light, 7 ACS Paragon Plus Environment


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i.e. mobile charges.33 SPV transient was performed with a vibrating probe and the light on/off was manually controlled using a small knob to open/close the shutter. To achieve better a signal-to-noise ratio, data points in the transient measurements were averaged and taken at 600 ms time interval, which sets the time resolution of the transient measurements.

Figure 1. (a) Schematic of surface photovoltage (SPV) measurement and sample structure. (b) AC surface photovoltage spectroscopy (SPS) – measurement with chopped monochromatic light and DC SPS – measurement with continuous illumination.

The complete OPV devices with HTL and metal contacts (100 nm Ag) were also made for external quantum efficiency (EQE), C-f and capacitance-voltage (C-V) measurements. EQE measurements were taken at short circuit using a monochromatic 8 ACS Paragon Plus Environment

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light from tungsten halogen lamp (Horiba TRIAX-180, grating 600 groove/mm) from 400 to 1000 nm. A chopper (Tetrahertz, C-995) was used to modulate the monochromatic light at 199 Hz, and a lock-in amplifier (Stanford Research System, SR830) was used to demodulate the signal. Cutoff filters at 710 and 850 nm were used to reduce the scattered light from the light source so that we can measure the weak sub-bandgap EQE response. EQE measurements were quantified using NREL calibrated Si photodiode. The capacitance measurements were performed using Zahner IM6 electrochemical workstation with the small AC bias (20 mV): C-f was measured at zero bias with frequency between 1 Hz and 1 MHz and C-V was measured at 1 kHz with DC bias ranging from -4.0 V to 0.5 V. Ionization energies (IE) of organic semiconductors were measured by photoelectron spectroscopy in air (PESA) (RKI instruments, AC2 model) with 100 nW deuterium lamp power. The resolution of PESA measurements is approximately 0.05 eV. UV-Vis absorption spectra of organic polymer and BHJ films were measured using an Ocean Optics 4000 spectrometer with a DT-mini-2-GS light source.



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3. RESULTS AND DISCUSSION

To show the difference between the two types of SPV measurement, we compare AC (taken at 200 Hz) and DC SPS of neat P3HT polymer on ZnO with P3HT absorption spectrum (Figure 2(a)). Each spectrum is normalized to its peak to highlight the spectral difference. The un-normalized data are shown in Figure S2. The sign of SPV depends on whether holes or electrons are transferred from the organic layer to the bottom transport layer: positive for hole transfer and negative for electron transfer. ZnO has a lower work function than P3HT, resulting in electrons transferring from P3HT to ZnO, leaving holes on the P3HT surface and yielding negative DC SPV signals. Our results are consistent with the published literature.18,28 AC SPS is similar to the absorption spectrum of P3HT with the same threshold at 1.95 eV, the optical bandgap of P3HT. Thus, AC SPS probes transitions from highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO). Figure S1(a) shows that the spectral shape of AC SPS is independent of chopping frequency from 50 Hz to 350 Hz, but the magnitude is maximum at 200-250 Hz (Figure S1(b)). DC SPS is broader compared to


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AC SPS with a tail extending towards lower energy and a threshold of 1.70 eV. Previously published DC SPS of P3HT26 also exhibits a 1.70 eV threshold. Thus, DC SPV signals arise from photo-induced transitions between HOMO and LUMO as well as from transitions between band states and sub-bandgap states, which can be defect states or band tail states. Because these states do not produce AC signals, they cannot respond to light modulation, i.e. they are slow trap states.

The location of these sub-bandgap states could be on the surface, in the bulk of P3HT, at the ZnO/P3HT interface, or in the ZnO. To differentiate between surface and bulk states, we deposited PEDOT:PSS on top of P3HT. Figure 2(b) shows the AC SPS, DC SPS and absorption spectrum of ZnO/P3HT/PEDOT:PSS: all three spectra are similar without the low-energy tail. To explicitly compare the effect of adding the PEDOT:PSS layer, we re-plot the data from Figure 2(a) and (b) in Figure 2(c) (AC SPS) and 2(d) (DC SPS) for the two samples. Depositing PEDOT:PSS on P3HT has little effect on AC SPS (Figure 2(c)), but has a strong effect on DC SPS. Figure 2(d) shows that DC SPS of the ZnO/P3HT (red) sample exhibits signals below the P3HT bandgap



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while that of ZnO/P3HT/PEDOT:PSS (blue) does not. The fact that these sub-bandgap SPV signals are not present after the deposition of PEDOT:PSS, we conclude that these traps are predominantly located on the surface of P3HT film. These results confirm that surface trap states are responsible for the SPV signals below absorption edge and can be removed with the deposition of PEDOT:PSS.


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Figure 2. Normalized comparison of AC and DC SPS of (a) ZnO/P3HT structure and (b) ZnO/P3HT/PEDOT:PSS structure with absorption spectrum (black dashed line in (a) and brown dashed line in (b)). (c) AC SPS and (d) DC SPS of ZnO/P3HT and ZnO/P3HT/PEDOT:PSS. Open symbols refer to AC SPS and closed symbols refer to DC SPS. Red color represents ZnO/P3HT sample and blue color represents ZnO/P3HT/PEDOT:PSS sample. Standard deviations of all measurements are smaller than the symbol sizes.

Next, we compare P3HT:PC61BM BHJ with neat P3HT. PEDOT:PSS HTL was deposited on both to elucidate the differences in SPV responses arising from the bulk of the organic layer. Figure 3(a) and (b) shows normalized AC and DC SPS of neat P3HT (blue) and P3HT:PC61BM (green), respectively. The un-normalized data are shown in Figure S3. It is clear that both AC and DC SPS are broader for P3HT:PC61BM BHJ, with a tail extended to low energies. While the threshold of ZnO/P3HT/PEDOT:PSS is 1.95 eV for both AC and DC SPS, that of ZnO/P3HT:PC61BM/PEDOT:PSS is ~ 1.70 eV in AC SPS and ≲ 1.5 eV in DC SPS. Note that we are not able to measure SPV signal for



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energies below 1.3 eV due to the low intensity of our light source. (Figure S4). The PC61BM excitonic feature at 1.75 eV34 is clearly visible in both AC and DC SPS of P3HT:PC61BM (black arrows in Figure 3(a) and (b)). In addition, CT states exist at donor-acceptor interfaces in BHJ, with energy corresponding to the difference between the PC61BM LUMO and the P3HT HOMO.34 To verify the existence of CT states in BHJ, we compare EQE spectra of P3HT and P3HT:PC61BM. Figure 3(c) shows that neat P3HT’s EQE spectrum (blue) drops steeply below the optical bandgap of P3HT (1.95 eV). In contrast, the EQE spectrum of P3HT:PC61BM BHJ (green) shows knee at ~ 1.75 eV, which has been attributed to PC61BM excitons,34 plus a gradual decrease towards low energies, corresponding to interfacial CT state contribution. From a gaussian fit to the low-energy EQE tail (dashed green line in Figure 3(c)),35 the interfacial CT state energy (ECT) is determined to be 1.11 eV for P3HT:PC61BM, in good agreement with the literature value (1.15 eV).34,35 Hence, the low-energy tail in the P3HT:PC61BM SPS is due to both PC61BM and interfacial CT states. Note that the reason that P3HT:PC61BM EQE spectrum appears to have a tail < 1.5 eV and SPS does not is due to that EQE signal is plotted in log scale and SPV in linear scale. Comparison of AC SPS, DC SPS, 14 ACS Paragon Plus Environment

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and EQE in linear scale is shown in Figure S5: EQE and AC SPS have the same threshold of 1.70 eV and the PC61BM excitonic feature is still clearly visible in the linear EQE spectrum.



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Figure 3. Normalized (a) AC SPS, (b) DC SPS, and (c) EQE of ZnO/P3HT/PEDOT:PSS (blue) and ZnO/P3HT:PC61BM/PEDOT:PSS (green). Open symbols refer to AC SPS and closed symbols refer to DC SPS. Blue and green curves and symbols represent P3HT and P3HT:PC61BM, respectively. The black arrows mark the PC61BM excitonic 16 ACS Paragon Plus Environment

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feature at 1.75 eV. The green dashed line shows the gaussian fit to CT state absorption to extract ECT.

To date, SPS studies have not been performed on conducting polymers other than P3HT or BHJ systems other than P3HT:PC61BM. Thus, we compare P3HT:PC61BM with another BHJ system, PFBT2Se2Th:PC71BM. Contribution from the surface is evaluated by depositing HTL on top of the organic layer. Figure 4(a) and (b) shows normalized DC SPS of P3HT:PC61BM (with and without PEDOT:PSS) and PFBT2Se2Th:PC71BM (with and without MoO3), respectively. The un-normalized data are shown in Figure S6. The DC SPS of bare BHJ are broader than those of HTL-covered BHJ for both systems. For P3HT:PC61BM, the SPS threshold with and without PEDOT:PSS are clearly red-shifted compared to the absorption spectrum (Figure 4(a) red dashed line). Additionally, there is a definite low-energy tail when the surface is not covered by PEDOT:PSS, similar to neat P3HT discussed above. It was established that a P3HT-rich layer exists on the surface of P3HT:PC61BM BHJ films,36,37,38 which as we discussed in Figure 2 has defect states in the gap. Note that our bare P3HT:PC61BM DC SPS results are similar to the



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published literature for energy > 1.3 eV.28 In contrast, DC SPS of PFBT2Se2Th:PC71BM does not exhibit a low-energy tail below the absorption threshold, and the spectra with or without HTL are similar, albeit the one without eMoO3 has a larger width. Thus, PFBT2Se2Th:PC71BM system has fewer optically active deep traps and the defect states do not concentrate on the surface of the organics.


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Figure 4. DC SPS of (a) ZnO/P3HT:PC61BM (closed black diamonds) and ZnO/P3HT:PC61BM/PEDOT:PSS (closed green circles) with fractional absorbance of P3HT:PC61BM (red dashed line), (b) ZnO/PFBT2Se2Th:PC71BM (closed pink triangles) and ZnO/PFBT2Se2Th:PC71BM/MoO3 (closed purple squares) with fractional absorbance of PFBT2Se2Th:PC71BM (orange dashed line). C-f results measured in the dark at zero bias and DOS derived from Eq. 1 as a function of energy with respect to HOMO position for (c) P3HT:PC61BM and (d) PFBT2Se2Th:PC71B M. The dashed line 19 ACS Paragon Plus Environment


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in (c) and (d) are the gaussian fits according to Eq. 2. Normalized SPV transients of (e) ZnO/P3HT:PC61BM (closed black squares) and ZnO/P3HT:PC61BM/PEDOT:PSS (closed green squares) measured at 2.25 eV and (f) ZnO/PFBT2Se2Th:PC71BM (closed pink triangles) and ZnO/PFBT2Se2Th:PC71BM/MoO3 (closed purple triangles) measured at 2 eV.

To understand SPS difference between the two BHJ systems, we further characterized deep level defects using C-f measurements. The solid line in Figure 4(c) and (d) shows the C-f spectrum of P3HT:PC61BM and PFBT2Se2Th:PC71BM in the dark, respectively. For both BHJ systems, C continuously increases below 100 Hz, indicating the presence of deep level defects. Density of defect states (DOS) distribution is proportional to the derivative of the capacitance with respect to the frequency as follows:39

𝑉𝑓𝑏 𝑑𝐶(𝜔)

𝐷𝑂𝑆(𝐸𝜔) = ― 𝑞𝑑𝑘𝐵𝑇

𝑑𝜔

,

(1)


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where q is the magnitude of elementary charge, d is the depletion width, ω=2πf is the angular frequency, kBT is the thermal energy, and Vfb is the flat band potential (Vfb ~ 0.42 V for P3HT:PC61BM and Vfb ~ 0.64 V for PFBT2Se2Th:PC71BM obtained from 1/C2-V plot in Figure S7). The open circles in Figure 4(c) and (d) shows the DOS distributions as a function of energy obtained from Eq. 1 for P3HT:PC61BM and PFBT2Se2Th:PC71BM, respectively. The DOS distribution is then fitted to a gaussian function:

𝐷𝑂𝑆 (𝐸) =

𝑁𝑡

[

― 2exp

2𝜋𝜎𝑡

(𝐸0 ― 𝐸)2 2𝜎2𝑡

],

(2)

where σt is the disorder width, Nt is the total defect density, and E0 is the defect energy at the peak of DOS. The C-f data show that the deep level defects in P3HT:PC61BM are centered at E0 ~ 0.47 eV with σt of 50 meV and those in PFBT2Se2Th:PC71BM are centered at E0 ~ 0.51 eV with σt of 40 meV. Since these conducting polymers are ptype, we assume the defects are hole traps near the HOMO.8 The HOMO positions (IE values) are 4.67 eV40 and 4.90 eV (Figure S8), and the optical band gaps are 2.0 eV 21 ACS Paragon Plus Environment


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and 1.6 eV for P3HT:PC61BM and PFBT2Se2Th:PC71BM, respectively. Combining these three energy values, we construct band diagrams for P3HT:PC61BM and PFBT2Se2Th:PC71BM, as shown in the insets of Figure 4(c) and (d), respectively.

Due to our SPV lamp intensity (Figure S4), we can only measure SPV signals for optical transitions > 1.3 eV. In the case of P3HT:PC61BM, transitions to LUMO from filled trap states with E0 ≲ 0.7 eV will produce a measurable SPV signal in our system; hence, the DC SPS threshold that begins at 1.3 eV is likely related to the deep trap states probed by C-f measurements. However, in the case of PFBT2Se2Th:PC71BM, E0 must be ≲ 0.3 eV to produce an SPV signal. Thus, the transitions from deep traps at E0 = 0.51 eV is too low in energy (~ 1.09 eV), and the PFBT2Se2Th:PC71BM SPS appears as if it does not have traps.

To examine the surface trap states in these BHJ systems, we compare the SPV transients for each BHJ with and without HTL (Figure 4(e) and (f)). Both systems respond to turning on light without measurable delay. However, when light is turned off at t = 0 sec, the SPV of ZnO/P3HT:PC61BM/PEDOT:PSS decays to zero within the 22 ACS Paragon Plus Environment

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measurement time resolution while that of ZnO/P3HT lasts for 36 seconds. Thus, these transient results further confirm that slow trap states exist at the surface of P3HT:PC61BM that can be removed by PEDOT:PSS deposition. In contrast, PFBT2Se2Th:PC71BM, SPV decays to zero with 9 sec (without MoO3) and 5.5 sec (with MoO3), consistent with defects reside mostly in the bulk and are not affected by HTL deposition.

4. CONCLUSION In this study, we used SPS to study optical transitions, both band-to-band and defect-to-band transitions, that produce charge transfer between an organic semiconductor (P3HT, P3HT:PC61BM and PFBT2Se2Th:PC71BM) and ZnO ETL. By comparing AC and DC spectra and the use of HTL deposition, we gain insight on the energetics and the physical location of defect states. For organics containing P3HT, 23 ACS Paragon Plus Environment


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defect states appear to be predominantly located on the surface. This is not the case in the PFBT2Se2Th:PC71BM BHJ. Since SPS relies on the separation of photogenerated charges, it can be used to study defects in the organic absorber as shown in this work, and can be extended to studying interfacial defects between the organic and the transport layers. Ultimately, SPS can be used to study charge transfer of a half solar cell before assembling the complete structure, which is useful for developing efficient OPV devices.

ASSOCIATED CONTENT

Supporting Information. AC SPS of ZnO/P3HT:PC61BM at different chopping frequencies. Un-normalized data of Fig. 2, 3, and 4. Intensity spectrum of SPV light source. EQE spectrum compared with AC and DC SPS of P3HT:PC61BM. Mott-Schottky analysis of P3HT:PC61BM and PFBT2Se2Th:PC71BM. PESA result of PFBT2Se2Th:PC71BM.


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AUTHOR INFORMATION

Corresponding Author * E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS We would like to thank Robert Piper for his assistance in the SPV transient measurements, and Trey B. Daunis, Rouzbeh M. Imenabadi, William G. Vandenberghe and Iain Baikie for useful discussion. This project is supported by National Science Foundation DMR-1305893 and Texas Photonic Center. JWPH acknowledges the Texas Instruments Distinguished Chair in Nanoelectronics.

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Probing Defect States in Organic Polymers and Bulk Heterojunctions

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